Atomic layer epitaxy (ALE) has become widely adopted as a method of providing atomic layer deposition and very thin film creation. The basic concept is a sequential exposure of a suitable surface to a precursor species followed by a co-reactant species. Originally, fluxes of two elements were alternated. Later, alternating molecular precursors were employed, which allowed expansion to several different co-reactant species. In both cases, reactants (elements and molecular precursors) are chemisorbed at each layer of creation. This resulted in self-limiting chemistry. Approaches with molecular precursors use reactants that are volatile, such as SiCl4 and H2O or where at least one co-reactant is in a gaseous state.
ALE may be considered a variant of the more general method of molecular beam epitaxy (MBE), where a thin film deposition process is controlled by flux intensity, time exposure, and temperature. Molecular beam epitaxy may be done in clusters, which may make control of the number of layers difficult. Atomic layer epitaxy may allow a layer-by-layer deposition, which may provide control over the number of layers deposited. Such molecular beam epitaxy does not always require chemisorption, but may instead use physisorption.
In semiconductor processing, barrier layers may be used to separate one material from another. For example, it may be desirable to use a barrier layer to separate a copper conductor layer from a silicon layer. Such barrier layers must be thick enough to provide complete separation, but thin enough to allow a more compact chip. To provide a barrier layer that is less than 100 nanometers instead of hundreds of nanometers, and provides complete separation is desirable. Present techniques for creating barrier layers, such as sputtering, chemical vapor deposition, and plasma enhanced chemical vapor deposition are normally used to create barrier layers that are hundreds of nanometers thick. Such barriers have a porosity that may require a thick layer. In addition, contaminants in the barrier layer may make the barrier layer less effective.
It would be desirable to create thin layers by an atomic layer deposition process that is an alternative to molecular beam epitaxy and atomic layer epitaxy. It would be desirable to create thin layers that have a high purity and low porosity, which may be more effective as barrier layers.